For the highly accurate interferometric measurement of a surface shape of a test object, such as a microlithographic optical element, diffractive optical arrangements are often used as what are known as null optics. In this case, the wavefront of a test wave is adapted to a desired shape of the surface by way of a diffractive element such that it would be perpendicularly incident on the desired shape at every location and be reflected back on itself thereby. Deviations from the desired shape can be determined by superposing a reference wave on the reflected test wave. The diffractive element used can be, for example, a computer-generated hologram (CGH).
DE 10 2012 217 800 A1 describes such a measurement arrangement having a complex coded CGH. A light wave is initially split into a reference wave and a test wave using a Fizeau element. The test wave is then converted by the complex coded CGH into a test wave having a wavefront that is adapted to the desired shape of the surface and calibration waves having a spherical or plane wavefront. To this end, the CGH has suitably configured diffractive structures. The calibration waves are used to calibrate the CGH. A test object is subsequently arranged in the test position, and a measurement using the test wave is performed. The test wave is reflected by the surface of the test object, transformed back by the CGH, and, after it passes through the Fizeau element, it is superimposed by the reference wave. It is possible to determine the shape of the surface from the interferogram captured in a plane. Here, a very high degree of accuracy is attained due to the calibration of the CGH.
However, one problem in measuring highly accurate surfaces using the known interferometric apparatuses is that a change in the optical properties of the CGH or other optical elements of the interferometer can occur between the calibration and the subsequent measurement of the test object. Such changes are caused in particular by temperature changes. In the case of a CGH with a quartz substrate, even inhomogeneous temperature changes in the mK range can cause a reduction in the measurement accuracy after a calibration.
This problem occurs in particular in the case of large and heavy test objects, such as for example microlithographic EUV mirrors. These test objects can be only slowly moved into a test position. In the process, a significant amount of energy and thus also heat is introduced into the measurement arrangement. Surfaces of optical elements having a significantly higher temperature with respect to the environment can also be determined only with insufficient measurement accuracy. Such optical elements could be employed in EUV microlithography due to desired power increases of the exposure radiation and an associated increased heating of the optical elements.